This invention relates to optical recording medium of the type wherein information is magneto-optically recorded and reproduced with the use of heat and light of a laser beam.
For optical recording media of magneto-optical memory type, there are well known a number of materials for a recording layer thereof, for example, MnBi, MnAlGe, MnSb, MnCuBi, GdFe, TbFe, GdCo, PtCo, TbCo, TbFeCo, GdFeCo, TbFeO.sub.3, GdIG (gadolinium iron garnet), GdTbFe, GdTbFeCoBi, CoFe.sub.2 O.sub.4, etc. These materials are deposited on transparent substrates of plastic material or glass as a thin film by any suitable thin-film forming techniques such as vacuum deposition or sputtering. The features common to these magneto-optical recording thin film layers are that the axis of easy magnetization is perpendicular to the film surface and that Kerr and Farady effects are great.
Requirements imposed on such magneto-optic recording media are:
(1) that the Curie point is of the order of 100.degree. to 200.degree. C. and the compensation point is close to room temperature, PA1 (2) that noise-inducing defects such as grain boundary are relatively fewer, and PA1 (3) that a magnetically and mechanically even film is attained over a relatively large area.
In the light of these requirements, a great attention is recently drawn to amorphous perpendicular magnetizable thin films of rare earth element-transition metal among the above-mentioned materials. Magneto-optical recording media having such amorphous perpendicular magnetizable thin films of rare earth element-transition metal, however, have a storage problem. If the magnetic thin film layers are stored in contact with the ambient atmosphere, rare earth elements therein are preferentially erroded or oxidized by oxygen and moisture in the atmosphere, losing the necessary information recording and reproducing ability.
The optical recording medium is also required to have as large a rotational angle upon recording and reproducing operation as possible in order to improve recording/reproducing performance and C/N (carrier-to-noise) ratio. A high reflectance of at least 20% is necessary to obtain a stable servo signal.
For this reason, most recording media are of the structure having an intermadiate layer formed between the substrate and the magnetic thin film layer or a protective layer on the magnetic thin film layer.
Intermediate layers for imparting corrosion resistance or moisture proofness and for adding multiple interference effect or Farady effect to Kerr effect to increase the rotational angle are disclosed in, for example, Japanese Patent Application Kokai No. 58-80142 as vacuum deposited films of inorganic materials such as silicon monoxide, silicon dioxide, aluminum nitride, silicon nitride and zinc sulfide. However, these layers are insufficient in corrosion resistance and other respects.
It is also known from Japanese Patent Application Kokai No. 60-145525 to form an intermediate layer from a mixture of oxide and nitride, more particularly a mixture of Si.sub.3 N.sub.4 and SiO.sub.2 and a mixture of AlN and Al.sub.2 O.sub.3. However, these layers are insufficient in C/N ratio, corrosion resistance and other respects.
We have proposed in Japanese Patent Application No. 61-370300 an intermediate layer which contains nitrides and oxides of at least two of metals and metalloids, for example, a mixture of Si, Al, N and O. However, these layers are insufficient in C/N ratio, corrosion resistance and other respects.
Japanese Patent Application Kokai No. 61-22458 discloses an intermediate layer which predominantly comprises silicon nitride and contains an additive ingredient such that the intermediate layer may have a refractive index of at least 2.1. A typical composition of the intermediate layer disclosed therein is Si.sub.3 N.sub.4 90 mol %-Al.sub.2 O.sub.3 6 mol %-Y.sub.2 O.sub.3 4 mol %, which has a Y content of 1.4 atom % and a (Si+N) content of 92.6 atom %.
Japanese Patent Application Kokai No. 61-278062 discloses an intermediate layer which is formed by sputtering a target containing silicon nitride, aluminum oxide, and yttrium oxide. The intermediate layer is disclosed therein as consisting of 0.1-5 mol % of Al.sub.2 O.sub.3, 0.1-3 mol % of Y.sub.2 O.sub.3, and 92-99.8 mol % of Si.sub.3 N.sub.4, which has a Y content of up to 0.9 atom % and a (Si+N) content of at least 94.2 atom %.
These intermediate layers have a too low reflectance to provide a stable servo signal.
These problems also occur in optical recording media having a recording layer of the so-called phase conversion type.